Bug 1478368 - [wdspec] Use a default window size of 800x600. r=ato

Previously we set this window size when restoring all default
values for a session during test teardown. Which means that
the very first test after the browser has been started will
run with the browser window default size.

This change enforces that all tests use the default window
size of 800x600.

MozReview-Commit-ID: Kiog3Ri2RJT

--HG--
extra : rebase_source : 971c1a17cdcc97d7880db8d8bdd50314369a2a91

testing/web-platform/tests/webdriver/tests/support/fixtures.py

@@ -19,6 +19,7 @@ default_script_timeout = 30
 default_page_load_timeout = 300
 default_implicit_wait_timeout = 0
 
+default_window_size = (800, 600)
 
 _current_session = None
 _custom_session = False
@@ -72,7 +73,7 @@ def cleanup_session(session):
         This also includes bringing it out of maximized, minimized,
         or fullscreened state.
         """
-        session.window.size = (800, 600)
+        session.window.size = default_window_size
 
     @ignore_exceptions
     def _restore_windows(session):
@@ -213,6 +214,9 @@ def session(capabilities, configuration, request):
         if not _current_session.session_id:
             raise
 
+    # Enforce a fixed default window size
+    _current_session.window.size = default_window_size
+
     yield _current_session
 
     cleanup_session(_current_session)